Sahih Muslim Book 4, Hadith 91
The Messenger of Allah (ﷺ) led the prayer and Abu Bakr was behind him. When the Messenger of Allah (ﷺ) recited the takbir, Abu Bakr also recited (it) in order to make it audible to us. And the rest of the hadith is like one transmitted by Laith.
The Messenger of Allah (ﷺ) was ill and we said prayer behind him and he was sitting. And Abu Bakr was making audible to the people his takbir. As he paid his attention towards us he saw us standing and (directed us to sit down) with a gesture. So we sat down and said our prayer with his prayer in a sitting posture. After uttering salutation he said: You were at this time about to do an act like that of the Persians and the Romans. They stand before their kings while they sit, so don't do that; follow your Imams. If they say prayer standing, you should also do so, and if they say prayer sitting, you should also say prayer sitting.
The Messenger of Allah (ﷺ) said: The Imam is appointed, so that he should be followed, so don't be at variance with him. Recite takbir when he recites it; bow down when he bows down and when he says:" Allah listens to him who praises Him," say:" O Allah, our Lord, to Thee be the Praise." And when he (the Imam) prostrates, you should also prostrate, and when he says prayer sitting, you should all observe prayer sitting.